A motor which is used for an acoustic device, such as a motor used for a VCR, is required to have rotational uniformity. It has also been desired to have the torque ripple generated by the motor be small. Concerning torque ripple, there are two kinds of torque ripple, namely cogging torque and a torque ripple which is generated on account of different torque constants depending on different positions of the permanent magnet rotor. The former is prevented from being generated by selection of a coreless motor. Considering the latter, there are two kinds of permanent magnet brushnless motors; one is a sinusoidal drive system transmitting current of 180 degrees per half cycle, and another is a pulsed drive system transmitting current of 120 degrees per half cycle. The former has an advantage of small torque ripple but has a drawback that the mechanism for outputting a sinusoidal signal is complex. At present, the latter is used more often. The pulsed drive system has a drawback in that the torque constant is varied according to the rotational position of the permanent magnet as explained above. An improved pulsed drive system is, for instance, disclosed on page 317, left column, the 4th paragraph of Japanese Patent Laid-Open No. 59-2556 published on Jan. 9, 1984 under the title of "DC motor". This system is characterized in that on an outer peripheral portion of a first disc-shaped permanent magnet, 2 m poles are magnetized at equal angle intervals. A second permanent magnet has an odd number multiple of poles of the first magnet connected at equal intervals. This prior-art system selects the the odd number multiple to be 5 or 7, to reduce the ripple of the driving torque constant. The principle of the prior-art is based on the ripple of the driving torque constant being reduced in such a manner that the torque constant of the 5th or 7th harmonic component is involved effectively to the driving coils.
However, in the prior-art, different permanent magnets having two polarities have to be magnetized into one permanent magnet. For manufacturing the dc motor of the prior-art, it is necessary that two magnetizing processes be carried out. After the first permanent magnets are magnetized, the second permanent magnets have to be magnetized to each permanent magnet. When the second magnets are going to be magnetized to the permanent magnets, a force for removing the magnetization tool is generated so that the second permanent magnets are not magnetized incorrectly, since the polarity of the second permanent magnets is the reverse of the first permanent magnets. For example, suppose that the first and second permanent magnets and the driving coils change the sinusoidal wave shape torque constant which corresponds to the peripheral position of the magnets; the variation of the torque constant is changed to 14% when only the first permanent magnets are magnetized on the outer peripheral of each permanent magnet and the accuracy of fitting the electric commutator is high; on the contrary the variation of the torque constant is changed to 3% when the first and second permanent magnets are magnetized on the outer peripheral of each permanent magnet at the same fitting accuracy of the first permanent magnets mentioned above. According to an experiment in an example in which the first permanent magnets have 8 poles and there are 6 driving coils, the variation of the torque constant becomes 3% when the position of each second permanent magnet is moved by 1 degree of mechanical angle to the pole of each first permanent magnet. On account of this, the characteristic of the motor is greatly changed. At the same time, there is a drawback in that the magnetizing tool of the second permanent magnets has to be accurate, since the area of the second permanent magnets is small and each area greatly affects the variation of the torque constant.